Abstract

We present a simple method for narrowing the intrinsic Lorentzian linewidth of a commercial ultraviolet grating extended-cavity diode laser (TOPTICA DL Pro) using weak optical feedback from a long external cavity. We achieve a suppression in frequency noise spectral density of 20 dB measured at frequencies around 1 MHz, corresponding to the narrowing of the intrinsic Lorentzian linewidth from 200 kHz to 2 kHz. Provided additional active low-frequency noise suppression and long-term drift compensation, the system is suitable for experiments requiring a tunable ultraviolet laser with narrow linewidth and low high-frequency noise, such as precision spectroscopy, optical clocks, and quantum information science experiments.

Figures (3)

Schematic diagram of the apparatus. The complete narrow linewidth system, consisting of the TOPTICA DL Pro supplemented by the long external cavity, is inside the dotted-line box, and the measurement chain is outside. The following abbreviations were used: PM for polarization-maintaining, PBS for polarizing beamsplitter, QWP for quarter-wave plate, PZT for piezoelectric transducer, and APD for avalanche photodiode.

Lorentzian linewidth as a function of feedback power ratio p from long external cavity using a 1 m fiber (blue) and a 4 m fiber (red). The linewidth narrowing is limited in both cases by multimode instability as p approaches the critical value of −39 dB. The top dash-dotted line represents the ECDL linewidth of 210 kHz without feedback from the long cavity, while the bottom dashed line represents the resolution limit due to amplitude noise. The FP used here has FWHM = 7.5 MHz.